Aqueous dispersions of polymeric materials (or emulsions having a dispersed phase of polymeric particles) produced by conventional emulsion polymerization techniques contain substantial quantities of protective colloids and surfactants. Conventional surfactants or emulsifiers and protective colloids are highly water sensitive, giving rise to water sensitivity in products formed from the emulsions, as discussed in more detail below, and hence are generally undesirable.
In various applications of latex polymers, the uniformity of particle size and average particle size play important roles. In order to produce emulsions of uniform particle size, using conventional emulsion polymerization procedures, it is necessary to resort to rather expensive and unconventional initiatiors.
In high gloss pigmented finishes, it is highly desirable that very small particle sized emulsions be used. In conventional emulsion polymerization, the actual size of the particle is generally controlled by the surfactant concentration. Thus, smaller particle sizes are obtained with higher surfactant concentrations and vice versa. However, higher concentrations of surfactant in an emulsion tend to render films formed therefrom highly water sensitive. Consequently, the much desired property of obtaining a fine particle sized emulsion that finds suitable application in high gloss water-based coating system has been obtained only with difficulty.
When such small particle sized emulsions are produced, not only is it necessary to employ large quantities of surfactants, but also it is necessary to introduce certain water-soluble colloids to stabilize the particles and prevent intraparticle coalescence. When such conventionally produced latex polymers are used in clear or pigmented air drying type finishes, the resulting coatings tend to be highly water sensitive. The reason for such water sensitivity stems from the high water solubility of the surfactant molecules and the water imbibing capacity of the protective colloids.
Conventional emulsion polymers are produced by free radical polymerization. When free radical polymerization is conducted with conventional water-soluble initiators, the pH of the reaction medium drifts toward acidic pH conditions. Acidic pH conditions are not favourable for emulsion polymerization since they lead to inefficiency, instability and coagulation. Consequently, it is conventional practice to use a series of buffering agents, such as phosphates and borates, to maintain the pH of the reaction medium at near neutral or slightly basic values. Buffering agents used are usually water soluble and their addition increases the water sensitivity of the films formed from emulsions containing them.
An additional drawback of conventional latex polymers is the constant drift in the pH of the emulsion, which again requires the addition of a buffering agent to achieve storage stability.
Free radial emulsion polymerization procedures result in high molecular weight polymeric products. The molecular weight of such polymers is normally in the range of 500,000 to a few million whereas the molecular weight of free-radially produced solution type polymers run in the 100,000 to 300,000 range. Tensile strength, impact strength, hardness, load bearing capacity, film forming and other mechanical properties are influenced by the polymer glass transition temperature and molecular weight. The mechanical properties of solution polymers are affected by the molecular weight. In addition, the polymer molecular weight influences the rheological properties of solution polymers. The solution polymers produced in the molecular weight range of 100,000 to 300,000 are good film formers and show adequate strength characteristics. Additionally, the solution polymers exhibit excellent rheological properties due to their relatively low molecular weight.
The latex polymers, due to the inherent nature of free radical polymerization, yield polymers of high molecular weight and hence exhibit superior mechanical strength characteristics. Unlike the solution polymers, the rheological properties of the polymers in the form of a latex are, generally, independent of the molecular weight. They are instead influenced by the latex particle size, particle size distribution, the number of particles and the type and concentration of emulsifier and protective colloids used in preparing the emulsion. As a result, the conventional latex polymers have poorer rheological properties than those of solution polymers.
In spite of the ease of emulsion polymerization, it is sometimes difficult to control the polymerization process, and hence the stability of the product, due to the rapid growth of the high molecular weight polymer particles. Consequently, to control the molecular weight of the latex polymers and hence the stability of the resultant latex particles, certain chain terminating agents and/or modifying agents, such as mercaptans and alcohols are used. It is highly undesirable to use such ingredients as the mercaptans and alcohols enhance the water sensitivity of the resultant polymer while both the alcohols and chlorinated solvents are pollution and health hazards.
One of the major application areas for latex polymers is in air drying clear and pigmented finishes of medium (70% on a 60.degree. angle gloss meter) and low (30% on a 60.degree. angle gloss meter) gloss. However, most latex polymers are unsuitable for producing high gloss pigmented finishes even at relatively low pigment levels. Additionally coatings made from these latex polymers exhibit very poor levelling. Organic solvent based finishes (enamel paints) such as the alkyds, urethanes, uralkyds and epoxies are capable of providing high gloss finishes which measure in the 90's on a 60.degree. angle gloss meter, and have excellent levelling and other application characteristics. However, since the organic solvents used in these coatings are often inflammable and toxic there is a definite need for suitable water based coatings capable of giving high gloss air drying finishes with acceptable application characteristics. It may be added that with some latex polymers high gloss values are attainable but only at very low pigment levels. The hiding power of the coatings at such pigment levels is unacceptably low.
In addition to the initial low gloss values of no more than about 70% on a 60.degree. angle gloss meter, finishes based on latex polymers tend to have very poor gloss retention, particularly in exterior use. This is perhaps due to the combination of large quantities of emulsifiers, surfactants, water-sensitive thickeners, colloids, pigment dispersion aids and other water-sensitive ingredients that are used in formulating the coating compositions.
As described earlier, latex polymers are sluggish in their flow behaviour, resulting in very poor levelling of the coating compositions. The flow properties of the coating are effected not only by those of the polymer particles themselves but also by those of other components of the composition such as water soluble thickeners. Latex polymer based pigmented or clear finishes need to be thickened to impart adequate film build and to impart thixotropy to prevent sagging. To achieve this thickening, it is normal practice to use cellulosic thickeners, which are readily available. Most cellulosic thickeners have acceptable flow characteristics only in limited applications. The use of cellulosic thickeners reduces the exterior durability of the finish as these thickeners, which are highly water-sensitive, tend to disintegrate under moisture and ultra-violet radiation. Some of them also undergo considerable yellowing on exterior exposure.
Consequently, a variety of synthetic polymeric thickeners has been developed that are suitable for exterior application. These polymeric thickeners, such as polyacrylates, although relatively high ultra-violet and water-resistant, do not have any better flow characteristics than the cellulosic thickeners. In addition, the thickening efficiency of such synthetic polymeric thickeners is much less than that of cellulosic thickeners, requiring the use of relatively large quantities. Consequently, when finishes containing such synthetic polymeric thickeners are used in exterior applications, the durability problem associated with cellulosic thickeners is largely overcome but the flow characteristics remain unsatisfactory.
In addition to the above drawbacks, conventional latex-based finishes have poor resistance to dirt pick-up, solvents, scrubbing and blocking and are softer than organic solvent-based enamels. Further, the water-based finishes have very poor resistance to fungal and microbiological attack.
It is common practice to produce finishes in a variety of colours and of varying intensities such as pastel shades, mid-tones and deep tone colours. With organic solvent-based finishes, it is possible to produce a range of colours without any detrimental effects on the gloss and gloss retention. However, with latex polymers insurmountable difficulties are encountered. For example, when tinting agents are added to a latex-based paint, generally the gloss drops drastically. This drop in gloss is mainly due to the incorporation of large quantities of pigment dispersants and tint acceptance aids. Such ingredients, in addition to affecting the gloss, reduce the mechanical strength and water-resistance of the finish.
With increasingly stringent regulations on the use of solvent-based finishes, industry is attempting to use, economically, water-based finishes in industrial applications. However, water-based finishes have considerable drawbacks and have yet to find wide spread use. One major problem is the production cycles. Water sensitive ingredients in latex polymers and the high molecular weight nature of the polymer result in the imbibing of large quantities of water which are retained for prolonged periods of time. Consequently when such latex polymers are used in industrial finishes, if an adequate drying or evaporation time is not provided prior to curing, the finishes tend to blister due to explosive evaporation of water and other volatile ingredients when high temperatures are encountered. Blistering can be avoided only by considerably prolonging the drying cycles, thus resulting in poor production efficiencies. To circumvent the latter problem, the industry has considered other water-based systems such as the water reducible and water thinnable alkyds, polyesters and the like. However, with such polymers, in addition to water, relatively large quantities of polar solvents, such as alcohols and amines need to be used, considerably increasing the cost of the process due to the necessity to recycle the solvents. In addition to these problems, latex-based finishes, when used in industrial application, suffer from other disadvantages such as lack of gloss, flow and hardness.
In electrodeposition using water-based systems, major surfactant and pigment migration problems are encountered resulting in non-uniform colours and inferior quality products. This necessitates constant monitoring and replenishment of the electrodeposition bath.
In view of the above prior art difficulties, in recent years considerable effort has been focussed on synthesizing aqueous dispersions of polymeric materials by processes involving only minor quantities of protective colloids and/or surfactants. However such procedures have not been successful in producing an emulsion having good overall properties.
Attempts have been made to incorporate specific molecules into the polymer to impart thereto a certain amount of surfactant properties. In this regard, stable aqueous dispersions of copolymers of ethylene with vinyl and acrylic monomers have been formed in the absence of conventional protective colloids and emulsifiers and in the presence of a copolymerizable half ester of maleic acid or maleic anhydride, the latter half ester providing the surfactant properties.
The latter procedure, however, uses very high pressure, thus requiring sophisticated equipment, and uses a predominance of ethylene in the copolymer to provide a variety of properties thereto. Additionally, the procedure requires the presence of substantial quantities of toxic solvents, such as tertiary butanol, to achieve solution of the water-insoluble reactants. The presence of such solvents, however, is known to affect the polymerization procedure drastically, resulting in high molecular weight products of large particle size.
In common with the conventional procedures described above, this prior art procedure also requires the presence of buffers in the aqueous medium to control pH during the polymerization reaction at near neutral or slightly alkaline pH. Further, this prior art procedure is only capable of producing dispersions having a low solids content from about 15 to 25% and it is necessary to concentrate the dispersion if a higher solids content is required, as is usually the case.
An alternative suggestion for making water based dispersions with low protective colloid and surfactant concentrations involves the formation of aqueous ammonical and/or alkaline dispersions of high molecular weight acrylic and/or vinyl polymers. The process for the manufacture of these products is very tedious and involves a multistep operation. Consequently these polymers are very expensive. Moreover, these dispersions contain large quantities of toxic alcohols and amines and are relatively low in polymer content (30 to 35%). Such dispersions have been in the market place for some time. However, their high cost coupled with their limited application potential restricting their use to only certain types of industrial finishes resulted in very little market penetration for such products.